How to observe the Sun

The bible tells "Light is sweet, and it is good for the eyes to see the Sun" (Ecclesiastics 11, v. 7), but it probably meant the enjoyment of sunlight, not looking directly at the undimmed Sun. Doing so, for even a brief moment, dazzles the eyes, and reveals no details of the Sun. Looking at the Sun for any length of time, or worse, through binoculars or a telescope, can seriously damage eyesight.

The safe method for observing the Sun was apparently introduced by Christopher Scheiner, one of the three observers who claimed to have discovered sunspots (Galileo and Fabricius were the others). He projected the Sun's magnified image from his telescope upon a flat white surface, and observed it there. That method is still the choice of observers. It is used by the world's greatest solar telescope, at Sacramento Peak ("Sac Peak") in New Mexico, which can throw a big image of the Sun onto a table in a cool underground room. The tube is fixed, built into the ground, while the Sun is tracked by a movable mirror at its top.

If you want to view the Sun--during an eclipse, for instance--do not look at it directly, but project its image onto a flat surface using a telescope (which may have a special attachment) or even a piece of cardboard with a pinhole. Alternatively, sun-filters exist for telescopes, and you may also look at the Sun through a totally blackened black-and-white film (old x-ray films may have suitable portions) or a welder's shield. Whatever you use, the Sun must appear comfortably dim.

On some rare occasion, the Sun near the horizon shines through a thick haze which dims it but does not make it appear less sharply. At such times large sunspots can be seen by the eye, and have in fact been reported long before Galileo by Chinese observers and others. But the Sun must appear dim, like an orange on the horizon.

How Far Away is the Sun?

The Sun's average distance from Earth, also known as the "astronomical unit" (AU), is about 150 million kilometers (93 million miles), but measuring that distance is not easy. By Kepler's 3rd law, if T is the orbital period of a planet and a its average distance from the Sun, then

T2 = k a3

where k is some number which is the same for all planets, including Earth. The exact value of k depends on the units used to measure T and a. Suppose T is measured in years and a in astronomical units (AU): then for the Earth, a = 1 and T = 1, telling us that in these units, k =1 and the equation reduces to

T2 = a3

By observing the motions of various planets, astronomers can readily obtain the value of T for each, from which a can then be calculated, giving planet's mean distance from the Sun in AU. We thus get a pretty good idea of the relative size of planetary orbits. But to know the actual distance in km or in miles, one of these distances, at the very least, must also be measured.

Given a map drawn on an unknown scale, we need only know the true value of any of its distances to calibrate all others. The same holds here: the distance from Earth to any planet is sufficient. When, for instance, radio signals were first bounced off the planet Venus from the giant radio-telescope at Arecibo, Puerto Rico (an immovable dish supported by a bowl-shaped valley), the time delay provided the value of the astronomical unit with greater accuracy than ever before. It is possible that even greater accuracy was obtained by tracking the radio signals of Voyager 2 as it passed Uranus and Neptune (giving a longer baseline), or of the Viking landers on Mars (giving a more sharply defined position)

Observations of the eclipsed Sun show the next layer above the photosphere--the reddish chromosphere ("chromos"=color), about 5000 km thick. Above that came the solar corona ("crown"), glowing tufts that faded into the distance, though time exposures with sensitive cameras managed to trace them to several solar radii.

Above the middle latitudes of the Sun the streaks of the corona sometimes displayed arches, suggesting that magnetic fields there determined its structure. That impression was reinforced by "polar plumes" above the north and south poles, spread out like the iron filings at the ends of a magnet, suggesting that the Sun, like the Earth, had two magnetic poles.

The heat of the Corona

The most interesting feature of the chromosphere and corona is that they are much hotter than the photosphere, even though any solar energy reaching them comes from the cooler region below.

The high temperature is inferred from the light emitted by their atoms. In a rarefied hot gas, emitted light no longer follows a simple pattern dictated by the temperature, like red-hot iron or a lightbulb filament (both of which are solid matter). Instead (see section S-4), it is concentrated in narrow ranges of color ("spectral lines") which depend on the atoms from which they came. Observations of emissions from atoms (or rather, ions) such as iron with 13 of its electrons missing suggest a coronal temperature of around 1,000,000°C (degrees Celsius), while emissions from chromospheric ions similarly suggest about 30,000°C.

The high temperature of the corona can also be seen from the x-rays and the extreme ultra-violet light which it emits, and indeed, images in these emissions (which must be taken outside the atmosphere) are now the preferred way of observing the corona (see section S-6). Such an image, color-coded to indicate regions of different temperature, is linked here. It shows that the heat is not evenly distributed:

The way the corona is heated is a mystery. It cannot be ordinary heat flow from the photosphere below, because ordinary heat radiation can never create a temperature higher than that of its source. Suppose you concentrate sunlight with magnifying glasses and mirrors, and suppose you somehow keep your sample from evaporating. You can never get it hotter than the Sun. As the sample heats up, it too radiates, and by the time its temperature reaches that of the Sun, it loses as much heat as it gains.

Some early theories proposed that the energy was coming from the outside, in the form of a steady rain of meteorites, greatly accelerated by the Sun's gravity, but that was soon disproved. Today it is believed that some plasma process is responsible, fed by local magnetic fields, but we really do not know much about that process. One idea is that it creates a sub-population of very fast ions, which do not share their energy because of the lack of collisions. At levels above the photosphere, solar gravity holds back the slower component, so that only very hot ions populate the higher layers, giving the corona a very high effective temperature.

Other theories suggest that plasma waves coming from the photosphere arrive in the corona and, unable to proceed into the rarefied gas, dissipate their energy. Because the gas receiving that energy is so rarefied, it heats up to a high temperature.

At the orbit of the Earth, the solar wind has an average density of about 6 ions/cm3, compared to 2.5 1019 molecules/cm3 in the Earth's sea-level atmosphere; it is more rarefied than the best laboratory vacuum. Earth receives solar wind from low and middle latitudes of the Sun, with an average outwards speed of 400 km/sec in the direction of Earth; above the Sun's poles the velocity nearly doubles, as reported by the Ulysses space probe. Past the Earth's orbit, the solar wind continues with undiminished speed (but with decreasing density, as its particles spread out), well past the orbit of Pluto. Scientists hope that Voyager 2, now racing away from the Sun, will still be transmitting when it crosses its outer boundary ("heliopause") or at least the "terminal shock, " a discontinuity preceding it, which should happen some time in the 21st century.